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Results: 1 through 43 of 43 (3.322 secs)
You searched for: creator:"N. Piskunov"
[texts]Radiative transfer in Doppler Imaging - N. Piskunov
The modern Doppler Imaging (DI) technique allows the reconstruction of different stellar surface structures based on accurate calculation of spectra of specific intensity. New applications like the mapping of the magnetic field vector put very stringent requirements on the radiative transfer (RT) solver which should be accurate, fast, and robust against numerical errors. We describe the evaluation of three different algorithms for our new magnetic DI code INVERS10...
Downloads: 4
[texts]Differential and Integral Calculus - N. Piskunov
Differential and Integral Calculus by N. Piskunov This text is designed as a course of mathematics for higher technical schools. It contains many worked examples that illustrate the theoretical material and serve as models for solving problems. The first two chapters "Number. Variable. Function" and "Limit. Continuity of a Function" have been made as short as possible. Some of the questions that are usually discussed in these chapters have been put in the third and subsequent chapters without lo...
Keywords: mathematics; calculus; integral; differential; mir publishers
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[texts]Magnetic Doppler Imaging of Active Stars - O. Kochukhov
We present a new implementation of the magnetic Doppler imaging technique, which aims at self-consistent temperature and magnetic mapping of the surface structures in cool active stars. Our magnetic imaging procedure is unique in its capability to model individual spectral features in all four Stokes parameters. We discuss performance and intrinsic limitations of the new magnetic Doppler imaging method...
Downloads: 2
[texts]DELO-Bezier formal solutions of the polarized radiative transfer equation - J. de la Cruz Rodríguez
We present two new accurate and efficient method to compute the formal solution of the polarized radiative transfer equation. In this work, the source function and the absorption matrix are approximated using quadratic and cubic Bezier spline interpolants. These schemes provide 2nd and 3rd order approximation respectively and don't suffer from erratic behavior of the polynomial approximation (overshooting)...
Downloads: 3
[texts]No magnetic field variation with pulsation phase in the roAp star gamma Equulei - O. Kochukhov
We present an analysis of 210 high-resolution time-resolved spectropolarimetric observations of the roAp star gamma Equ obtained over three nights in August and September 2003. Radial velocity variations due to p-mode non-radial pulsations are clearly detected in the lines of rare-earth elements, in particular Pr III, Nd II and Nd III. In contrast, we find absolutely no evidence for the variation of the mean longitudinal magnetic field over the pulsation period in gamma Equ at the level of 110-2...
Downloads: 7
[texts]Least squares deconvolution of the stellar intensity and polarization spectra - O. Kochukhov
Least squares deconvolution (LSD) is a powerful method of extracting high-precision average line profiles from the stellar intensity and polarization spectra. Despite its common usage, the LSD method is poorly documented and has never been tested using realistic synthetic spectra. In this study we revisit the key assumptions of the LSD technique, clarify its numerical implementation, discuss possible improvements and give recommendations how to make LSD results understandable and reproducible...
Downloads: 2
[texts]Planet formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks - W. Lyra
As accretion in protoplanetary disks is enabled by turbulent viscosity, the border between active and inactive (dead) zones constitutes a location where there is an abrupt change in the accretion flow. The gas accumulation that ensues triggers the Rossby wave instability, that in turn saturates into anticyclonic vortices. It was suggested that the trapping of solids within them leads to a burst of planet formation on very short timescales...
Downloads: 1
[texts]Reducing low signal-to-noise FUSE spectra: confirmation of Lyman continuum escape from Haro 11 - E. Leitet
Galaxies are believed to be the main providers of Lyman continuum (LyC) photons during the early phases of the cosmic reionization. Little is known however, when it comes to escape fractions and the mechanisms behind the leakage. To learn more one may look at local objects, but so far only one low-z galaxy has shown any sign of emitting LyC radiation. With data from the Far Ultraviolet Spectroscopic Explorer (FUSE), Bergvall et al...
Downloads: 3
[texts]Rotation, magnetism, and metallicity of M dwarf systems - D. Shulyak
Close M-dwarf binaries and higher multiples allow the investigation of rotational evolution and mean magnetic flux unbiased from scatter in inclination angle and age since the orientation of the spin axis of the components is most likely parallel and the individual systems are coeval. Systems composed of an early (M0.0 -- M4.0) and a late (M4.0 -- M8.0) type component offer the possibility to study differences in rotation and magnetism between partially and fully convective stars...
Downloads: 2
[texts]3-D hydrodynamic simulations of convection in A stars - O. Kochukhov
Broadening and asymmetry of spectral lines in slowly rotating late A-type stars provide evidence for high-amplitude convective motions. The properties of turbulence observed in the A-star atmospheres are not understood theoretically and contradict results of previous numerical simulations of convection. Here we describe an ongoing effort to understand the puzzling convection signatures of A stars with the help of 3-D hydrodynamic simulations...
[texts]Inhomogeneous distribution of mercury on the surfaces of rapidly rotating HgMn stars - O. Kochukhov
Starspots are usually associated with the action of magnetic fields at the stellar surfaces. However, recently an inhomogeneous chemical distribution of mercury was found for the mercury-manganese (HgMn) star alpha And -- a well-established member of a non-magnetic subclass of the chemically peculiar stars of the upper main sequence. In this study we present first results of the high-resolution survey of the HgII 3984 resonance line in the spectra of rapidly rotating HgMn stars with atmospheric ...
[texts]Embryos grown in the dead zone: Assembling the first protoplanetary cores in low mass self-gravitating circumstellar disks of gas and solids - W. Lyra
In the borders of the dead zones of protoplanetary disks, the inflow of gas produces a local density maximum that triggers the Rossby wave instability. The vortices that form are efficient in trapping solids. We aim to assess the possibility of gravitational collapse of the solids within the Rossby vortices. We perform global simulations of the dynamics of gas and solids in a low mass non-magnetized self-gravitating thin protoplanetary disk with the Pencil code...
Downloads: 1
[texts]Linear spectropolarimetry of Ap stars: a new degree of constraint on magnetic structure - G. A. Wade
We present preliminary results from a programme aimed at acquiring linear spectropolarimetry of magnetic A and B stars. Linear polarization in the spectral lines of these objects is due to the Zeeman effect, and should provide detailed new information regarding the structure of their strong magnetic fields. To illustrate the impact of these new data, we compare observed circular and linear polarization line profiles of 53 Cam with the profiles predicted by the magnetic model by Landstreet (1988)...
Downloads: 2
[texts]Eu III identification and Eu abundance in cool CP stars - T. Ryabchikova
We report the first identification of the Eu III lambda 6666.317 line in optical spectra of CP stars. This line is clearly present in the spectra of HR 4816, 73 Dra, HR 7575, and beta CrB, while it is marginally present or absent in spectra of the roAp stars alpha Cir, gamma Equ, BI Mic, 33 Lib, and HD 24712.
Downloads: 3
[texts]iota Cas: Multi-element Doppler imaging and magnetic field geometry - R. Kuschnig
In order to clarify the role of the magnetic field in generating abundance inhomogeneities in the atmospheres of Ap stars, we present new abundance Doppler images and an approximate magnetic field geometry for the Ap star iota Cas.
Downloads: 3
[texts]Self-broadening in Balmer line wing formation in stellar atmospheres - P. S. Barklem
Details of a theory of self-broadening of hydrogen lines are presented. The main features of the new theory are that the dispersive-inductive components of the interaction (van der Waals forces) have been included, and the resonance components have been computed by perturbation theory without the use of the multipole expansion. The theory is applied to lower Balmer lines and the theoretical and observational impact of the new broadening theory is examined...
Downloads: 1
[texts]Standing on the shoulders of giants: Trojan Earths and vortex trapping in low mass self-gravitating protoplanetary disks of gas and solids - W. Lyra
Centimeter and meter sized solid particles in protoplanetary disks are trapped within long lived high pressure regions, creating opportunities for collapse into planetesimals and planetary embryos. We study the accumulations in the stable Lagrangian points of a giant planet, as well as in the Rossby vortices launched at the edges of the gap it carves. We employ the Pencil Code, tracing the solids with a large number of interacting Lagrangian particles, usually 100,000...
Downloads: 6
[texts]Global magnetohydrodynamical models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and transport of solids - W. Lyra
We present global 3D MHD simulations of disks of gas and solids, aiming at developing models that can be used to study various scenarios of planet formation and planet-disk interaction in turbulent accretion disks. A second goal is to show that Cartesian codes are comparable to cylindrical and spherical ones in handling the magnetohydrodynamics of the disk simulations, as the disk-in-a-box models presented here develop and sustain MHD turbulence...
Downloads: 3
[texts]Magnetic fields in M-dwarfs: quantitative results from detailed spectral synthesis in FeH lines - D. Shulyak
Strong surface magnetic fields are ubiquitously found in M-dwarfs with mean intensities on the order of few thousand Gauss-three orders of magnitude higher than the mean surface magnetic field of the Sun. These fields and their interaction with photospheric convection are the main source of stellar activity, which is of big interest to study links between parent stars and their planets. Moreover, the understanding of stellar magnetism, as well as the role of different dynamo-actions in particula...
[texts]Time-resolved photometric and spectroscopic analysis of a luminous Ap star HD103498 - S. Joshi
We present the results on the photometric and spectroscopic monitoring of a luminous Ap star HD103498. The time-series photometric observations were carried out on 17 nights using three-channel fast photometer attached to the 1.04-m optical telescope at ARIES, Nainital. The photometric data of five nights of year 2007 show clear signature of 15-min periodicity. However, the follow-up observations during 2007--2009 could not repeated any such periodicity...
Downloads: 2
[texts]Multi-element Doppler imaging of kappa Psc - N. Piskunov
kappa Psc (HD 220825) is a typical Chromium Ap star that happens to have optimal parameters for Doppler imaging (DI). Its short rotational period of less then 2 days, rotational velocity of ~40 km/s, and a moderate inclination of the rotational axis put modest requirements on spectroscopic observations. Anomalies of iron peak elements are clearly present, but small enough to cause significant deviations from model atmospheres with scaled solar abundances...
Downloads: 2
[texts]Magnetic Doppler imaging of the roAp star HD 24712 - T. Lüftinger
We present the first magnetic Doppler images of a rapidly oscillating Ap (roAp) star. We deduce information about magnetic field geometry and abundance distributions of a number of chemical elements on the surface of the hitherto best studied roAp star, HD 24712, using the magnetic Doppler imaging (MDI) code, INVERS10, which allows us to reconstruct simultaneously and consistently the magnetic field geometry and elemental abundance distributions on a stellar surface...
Downloads: 2
[texts]Modelling the molecular Zeeman effect in M-dwarfs: methods and first results - D. Shulyak
We present first quantitative results of the surface magnetic field measurements in selected M-dwarfs based on detailed spectra synthesis conducted simultaneously in atomic and molecular lines of the FeH Wing-Ford $F^4\,\Delta-X^4\,\Delta$ transitions. A modified version of the Molecular Zeeman Library (MZL) was used to compute Land\'e g-factors for FeH lines in different Hund's cases. Magnetic spectra synthesis was performed with the Synmast code...
Downloads: 2
[texts]A probable stellar solution to the cosmological lithium discrepancy - A. J. Korn
The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision. The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old star...
Downloads: 1
[texts]Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397 - A. J. Korn
We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] = -2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar-parameter differences between groups of stars, are in good agreement with broad-band and Stroemgren photometry calibrated on the infrared-flux method...
Downloads: 2
[texts]Detailed analysis of Balmer lines in cool dwarf stars - P. S. Barklem
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and subgiant stars is presented using MARCS model atmospheres based on the most recent calculations of the line opacities. A detailed quantitative comparison of the solar flux spectra with model spectra shows that Balmer line profile shapes, and therefore the temperature structure in the line formation region, are best represented under the mixing length theory by any combination of a low mixing-length parameter alpha and a l...
Downloads: 2
[texts]First detection of linear polarization in the line profiles of active cool stars - O. Kochukhov
The application of high-resolution spectropolarimetry has led to major progress in understanding the magnetism and activity of late-type stars. During the last decade, magnetic fields have been discovered and mapped for many types of active cool stars using spectropolarimetric data. However, these observations and modeling attempts are fundamentally incomplete since they are based on the interpretation of the circular polarization alone...
Downloads: 2
[texts]The search for magnetic fields in mercury-manganese stars - V. Makaganiuk
We performed a highly sensitive search for magnetic fields on a large set of HgMn stars. With the aid of a new polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we obtained high-quality circular polarization spectra of 41 single and double HgMn stars. Using a multi-line analysis technique on each star, we co-added information from hundreds of spectral lines resulting in significantly greater sensitivity to the presence of magnetic fields, including very weak fields...
[texts]Magnetism, chemical spots, and stratification in the HgMn star phi Phoenicis - V. Makaganiuk
Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots...
Downloads: 8
[texts]Are there tangled magnetic fields on HgMn stars? - O. Kochukhov
Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects. We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of 8 slowly rotating HgMn stars and one normal late-B star...
Downloads: 13
[texts]Multi-element Doppler imaging of the CP2 star HD 3980 - N. Nesvacil
In atmospheres of magnetic main-sequence stars, the diffusion of chemical elements leads to a number of observed anomalies, such as abundance spots across the stellar surface. The aim of this study was to derive a detailed picture of the surface abundance distribution of the magnetic chemically peculiar star HD 3980. Based on high-resolution, phase-resolved spectroscopic observations of the magnetic A-type star HD 3980, the inhomogeneous surface distribution of 13 chemical elements (Li, O, Si, C...
Downloads: 3
[texts]No magnetic field in the spotted HgMn star mu Leporis - O. Kochukhov
Chemically peculiar stars of the mercury-manganese (HgMn) type represent a new class of spotted late-B stars, in which evolving surface chemical inhomogeneities are apparently unrelated to the presence of strong magnetic fields but are produced by some hitherto unknown astrophysical mechanism. The goal of this study is to perform a detailed line profile variability analysis and carry out a sensitive magnetic field search for one of the brightest HgMn stars - mu Lep...
Downloads: 8
[texts]Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani - V. Makaganiuk
According to our current understanding, a subclass of the upper main sequence chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were recently discovered on their surfaces. At the same time, no global magnetic fields stronger than 1-100 G are detected by modern studies. The goals of our study are to search for magnetic field in the HgMn binary system 66 Eri and to investigate chemical spots on the stellar surfaces of both component...
Downloads: 7
[texts]Study of the $\bm{p+^6}$Li$\bm{\toη+^7}$Be reaction 11.3 MeV above threshold - A. Budzanowski
The cross section for the reaction $p+^6\text{Li}\to\eta+^7\text{Be}$ was measured at an excess energy of 11.28 MeV above threshold by detecting the recoiling $^7$Be nuclei. A dedicated set of focal plane detectors was built for the magnetic spectrograph Big Karl and was used for identification and four momentum measurement of the $^7$Be. A differential cross section of $\frac{d\sigma}{d\Omega}=(0.69\pm{0.20}\text{(stat.)}\pm 0.20\text{(syst.)})\text{nb/sr}$ for the ground state plus the 1/2$^-$...
Downloads: 1
[texts]A large acceptance scintillator detector with wavelength shifting fibre read-out for search of eta-nucleus bound states - K. Kilian
A large acceptance scintillator detector with wavelength shifting optical fibre readout has been designed and built to detect the decay particles of $\eta$-nucleus bound system (the so-called $\eta$-mesic nuclei), namely, protons and pions. The detector, named as ENSTAR detector, consists of 122 pieces of plastic scintillator of various shapes and sizes, which are arranged in a cylindrical geometry to provide particle identification, energy loss and coarse position information for these particle...
Downloads: 1
[texts]Upper limits for a narrow resonance in the reaction p + p -> K^+ + (Lambda p) - A. Budzanowski
The reaction pp -> K^+ + (Lambda p) has been measured at T_p = 1.953 GeV and \Theta = 0 deg with a high missing mass resolution in order to study the Lambda p final state interaction. Narrow S = -1 resonances predicted by bag model calculations are not visible in the missing mass spectrum. Small structures observed in a previous experiment are not confirmed. Upper limits for the production cross section of a narrow resonance are deduced for missing masses between 2058 and 2105 MeV/c^2.
Downloads: 1
[texts]High resolution study of the Lambda p final state interaction in the reaction p + p -> K+ + (Lambda p) - A. Budzanowski
The reaction pp -> K+ + (Lambda p) was measured at Tp=1.953 GeV and Theta = 0 deg with a high missing mass resolution in order to study the Lambda p final state interaction. The large final state enhancement near the Lambda p threshold can be described using the standard Jost-function approach. The singlet and triplet scattering lengths and effective ranges are deduced by fitting simultaneously the Lambda p invariant mass spectrum and the total cross section data of the free Lambda p scattering.
Downloads: 8
[texts]Cross section of the $pp\to K^+Σ^+n$ reaction close to threshold - A. Budzanowski
We have measured inclusive data on $K^+$-meson production in $pp$ collisions at COSY J\"ulich close to the hyperon production threshold and determined the hyperon-nucleon invariant mass spectra. The spectra were decomposed into three parts: $\Lambda p$, $\Sigma^0p$ and $\Sigma^+n$. The cross section for the $\Sigma^+n$ channel was found to be much smaller than a previous measurement in that excess energy region...
Downloads: 13
[texts]Cross Section and Tensor Analysing Power of the $\vec{d}d\to ηα$ Reaction Near Threshold - The GEM Collaboration
The angular distributions of the unpolarised differential cross section and tensor analysing power $A_{xx}$ of the $\vec{d}d\to\alpha \eta$ reaction have been measured at an excess energy of 16.6 MeV. The ambiguities in the partial-wave description of these data are made explicit by using the invariant amplitude decomposition. This allows the magnitude of the s-wave amplitude to be extracted and compared with results published at lower energies...
Downloads: 3
[texts]Precision measurements of the pp\to π^+pn and pp\to π^+d reactions: importance of long-range and tensor force effects - A. Budzanowski
Inclusive measurements of pion production in proton--proton collisions in the forward direction were undertaken at 400 and 600 MeV at COSY using the Big Karl spectrograph. The high resolution in the $\pi^+$ momentum ensured that there was an unambiguous separation of the $pp\to {\pi}^+d/\pi^+pn$ channels. Using these and earlier data, the ratio of the production cross sections could be followed through the $\Delta$ region and compared with the predictions of final state interaction theory...
Downloads: 1
[texts]A precision determination of the mass of the $η$ meson - The GEM collaboration
Several processes of meson production in proton-deuteron collisions have been measured simultaneously using a calibrated magnetic spectrograph. Among these processes, the $\eta$ meson is seen clearly as a sharp missing--mass peak on a slowly varying background in the $p+d\to ^3\textrm{He} +X$ reaction. Knowing the kinematics of the other reactions with well determined masses, it is possible to deduce a precise mass for the $\eta$ meson...
Downloads: 1
[texts]ABC Effect in Basic Double-Pionic Fusion --- Observation of a new resonance? - P. Adlarson
We report on a high-statistics measurement of the basic double pionic fusion reaction $pn \to d\pi^0\pi^0$ over the energy region of the so-called ABC effect, a pronounced low-mass enhancement in the $\pi\pi$-invariant mass spectrum. The measurements were performed with the WASA detector setup at COSY. The data reveal the ABC effect to be associated with a Lorentzian shaped energy dependence in the integral cross section...
Downloads: 3
[texts]Search for a dark photon in the $π^0 \to e^+e^-γ$ decay - WASA-at-COSY Collaboration
The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->pp\pi^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV
Downloads: 8
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